Skywatchers have a double treat in store on 27 July: the longest total lunar eclipse of the 21st century and Mars at its brightest for many years. The red planet and the (temporarily) red Moon will appear together in the same part of the sky after sunset. Mars, the Earth, and the Sun will be roughly lined up on 27 July. Mars is on the opposite side of the Earth to the Sun, and hence the alignment is known as opposition. This coincides with the time when the planet is near its closest point to the Earth, and that night it will be about 57.7 million kilometres away. This means that Mars also appears near its largest apparent size in a telescope and is close to its maximum apparent brightness.

Mars has a pronounced elliptical orbit, which means its distance from the Sun varies significantly. The distance between the red planet and the Earth at opposition also changes: from less than 56 million kilometres (most recently in 2003) to around 101 million kilometres (Mars was this far away at opposition in 2012). On 27 July the red planet will rise in the south-eastern sky at 21:34 BST in London, and 22:20 in Glasgow. In the weeks after opposition the view is almost as good, so if bad weather spoils the view there is still plenty of time to see Mars at its best. The planet actually reaches its closest point to the Earth on 30 July, when it will be 57.6 million kilometres away.

Map of the Mekong Delta in Vietnam and Cambodia. A narrow trough of theancient South China Sea covered land north of Phnom Penh creating the deltalowlands. Map courtesy of Mic Greenberg. (111)

The Mekong Delta is home to 15 million people, many of whom rely on the delta’s rich soil and water resources for farming and fishing. But their livelihoods are being threatened by rising sea levels, droughts, dams, and other hydrological shifts. A new article from researchers at the University of Illinois and Iowa State University explains the challenges. “The management dilemma for the Mekong Delta is to ensure a habitable environment for human well-being and for rice and aquaculture productivity, while strategically conserving wetland ecologies,” saysKenneth Olson, professor emeritus in the Department of Natural Resources and Environmental Sciences at U of I and co-author of the article.

The delta, which sits at the southern tip of Vietnam and Cambodia, is characterized by seasonal flooding and nutrient-rich sediment deposits from the Mekong and Bassac Rivers as they flow toward the South China Sea. Historically, mosquitoes, malaria, waterborne diseases, and unpredictable flow patterns made the delta a difficult place to live.

Zika virus (ZIKV) may be sexually transmissible for a shorter period than previously estimated, according to a systematic review published this week in PLOS Medicine by Michel Counotte and Nicola Low of the University of Bern in Switzerland, and colleagues from the World Health Organization, US Centers for Disease Control and National Institute of Allergy and Infectious Disease. The review included analysis of data from both human and animal studies, and was conducted to describe the epidemiology of sexual transmission of ZIKV.

Sexual transmission of ZIKV has been previously documented, but the risks of transmission are not well understood, and it is not known whether other flaviviruses can be transmitted this way. To address this gap in knowledge, the researchers conducted a systematic review of available, relevant evidence through 15 April 2018. Counotte and colleagues found that, where documented, sexual transmission of ZIKV is much more common from men to women than from women to men. For sexual transmission of ZIKV, the authors found the median serial interval – the time between onset of symptoms in 2 sexual partners –is 12 days, and the median duration of ZIKV RNA persistence in semen is longer (34 days) than in the female genital tract (12 days). They found no evidence of sexual transmission for any other arthropod-borne flaviviruses.